1. Field of the Invention
The present invention relates to voltage regulator circuits. More particularly, the invention relates to a multi-phase power converter that can operate at high switching frequencies with effective ripple cancellation and reduced switching loss.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A synchronous buck converter is a particular type of switched mode converter that uses two power switches, typically MOSFET transistors, to control the flow of current in the output inductor. A high-side switch selectively couples the inductor to a first power supply voltage while a low-side switch selectively couples the inductor to a second power supply voltage, such as ground. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the high-side and low-side switches in an alternating manner. Synchronous buck converters generally offer high efficiency and high power density, particularly when MOSFET devices are used due to their relatively low on-resistance.
For certain applications having demanding current requirements, it is known to combine plural synchronous buck converter modules together in multi-phase configurations operated in an interleaf mode. The output inductors of each of the buck converter modules are connected together to provide a single output voltage. The PWM control circuit provides a variable duty cycle control signal to each buck converter module in order to control its switching. The multiple modules are operated in a synchronous manner, with the respective high-side switches of each channel being switched on at different phases of a power cycle. Interleaf operation is advantageous in that it reduces the current ripple across the filter capacitor and makes the ripple frequency a multiple of the switching frequency, thereby enabling the use of smaller filter capacitors to reduce the ripple. Also, by spreading the output current among the multiple channels, the thermal load on the power semiconductor components of the power converter is reduced.
Recent advancements in microprocessors continue to drive a demand for power converters that supply increasingly low output voltages (e.g., less than 1.5 volts) at high load current (e.g., greater than 40 amps). To satisfy this demand, multi-phase power converters are operated at very high switching frequencies (e.g., greater than 100 kHz). But, at high switching frequencies, as the duty cycle is made very small (e.g., 10-40%), multi-phase power converters tend to exhibit poor ripple cancellation. Moreover, the high-side MOSFET devices have high switching losses due to the high switching voltage and current. To solve these problems, power converter topologies that utilize groups of coupled magnetic configurations have been proposed, such as the coupled buck converter and the tapped inductor buck converter. These topologies extend the power converter duty cycle, and have better ripple cancellation and lower switching losses due to lower switching current.
Nevertheless, these coupled magnetic topologies also have other drawbacks that make them less attractive as alternative designs. Coupled magnetic configurations are not functionally equivalent to buck converters, and operate analogously to flyback converter topologies in that the load current is partly supplied by the filter capacitor during the on-time of the power switches, hence requiring a larger filter capacitor. Further, coupled magnetic configurations have stability problems due to a right half plane zero that introduces an extra phase-shift of 90xc2x0 into the control loop. Coupled magnetic configurations also have poor efficiency and large output ripple due to their discontinuous energy transfer to the output inductor. Lastly, coupled magnetic configurations use a single ended inductor in which the magnetic flux swing is unidirectional. This results in poor utilization of the transformer core since the core flux must reset naturally. In addition to these drawbacks, the relationship between the inductor turns ratio and duty cycle is not linear, so the duty cycle cannot be increased proportionally with the inductor turns ratio.
Accordingly, it would be desirable to provide a multi-phase power converter that can operate at high switching frequencies with effective ripple cancellation and reduced switching loss for high power density converter applications. It would also be desirable to provide such a multi-phase power converter having a simple, true buck-derived topology.
The present invention overcomes these drawbacks of the prior art by providing a multi-phase power converter that can operate at high switching frequencies with effective ripple cancellation and reduced switching loss for high power density converter applications.
In an embodiment of the invention, the multiple-phase power converter comprises a non-isolated, double-ended transformer having a plurality of windings, a high-side switch portion, a low-side switch portion, an output portion, and a controller. The high-side switch portion includes a first power switch connecting an input voltage source (VIN) to a virtual phase node through a first winding of the plurality of windings and a second power switch connecting the input voltage source to the virtual phase node through a second winding of the plurality of windings. The first and second windings are arranged with opposite polarity. The low-side switch portion includes a third power switch connecting the virtual phase node to ground through a third winding of the plurality of windings and a fourth power switch connecting the virtual phase node to ground through a fourth winding of the plurality of windings. The third and fourth windings are arranged with opposite polarity. The output portion includes an output inductor connecting the virtual phase node to an output terminal providing an output voltage (VOUT).
The controller is adapted to control operations of the first, second, third and fourth power switches such that the first and second power switches are enabled in respective alternating phases. The third switch is disabled when the second switch is enabled so that current flows concurrently through both the second and fourth windings to the output inductor during a first phase, and the fourth switch is disabled when the first switch is enabled so that current flows concurrently through the first and third windings to the output inductor. In a preferred embodiment, the first and second power switches are driven by respective control signals having a duty cycle of approximately 25% and a relative phase difference of 180xc2x0. Likewise, the third and fourth power switches are driven by respective control signals having a duty cycle of approximately 75% and a relative phase difference of 180xc2x0.
The multiple-phase power converter may further include a snubber circuit electrically connected to the high-side portion to reduce voltage ringing or spikes due to leakage inductance of the double-ended transformer. The snubber circuit may further comprise a low pass filter electrically connected to the first and second power switches. Alternatively, the snubber circuit may comprise a diode and a parallel-connected resistor and capacitor connected electrically between each of the first and second power switch or across each of the first and second winding. The multiple-phase power converter may further comprise a capacitor electrically connected between the first and second power switches to provide clamping of the high-side portion.
In another embodiment of the invention, the multiple-phase power converter is adapted for four-phase operation, and comprises a non-isolated, double-ended transformer having a plurality of windings, a first high-side switch portion, a first low-side switch portion, a first output portion, a second high-side switch portion, a second low-side switch portion, a second output portion and a controller. The first, second, fifth and sixth power switches are driven by respective control signals having a duty cycle of approximately 25% and a relative phase difference of 90xc2x0. The third, fourth, seventh and eighth power switches are driven by respective control signals having a duty cycle of approximately 75% and a relative phase difference of 90xc2x0.
A more complete understanding of the multi-phase power converter will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.